1. Field of the Invention
The present invention relates to a battery current sensing device having a current sensor electrically connected to the battery of a vehicle and having a measuring circuit on a printed circuit board which is electrically and mechanically connected in a fixed position relative to the current sensor such that the measuring circuit is electrically connected to the current sensor.
2. Background Art
DE 199 61 311 A1 (corresponds to U.S. Pat. No. 6,787,935) describes a battery current sensing device which is mechanically mounted on a pole terminal of a battery of a vehicle. The device includes a current sensor having a measuring resistor between contact surfaces of the sensor. Soldered connections electrically and mechanically connect the sensor to a printed circuit board (“PCB”). The contact surfaces of the sensor are made of copper and the measuring resistor is made of manganin (i.e., an alloy of copper, manganese, and nickel). The sensor and the PCB have similar thermal expansion coefficients.
A problem is that the measuring resistor of the current sensor of a battery current sensing device is subjected to severe temperature fluctuations during vehicle operation. For example, the measuring resistor is cold when the vehicle has been parked in cold temperature for a long time. Upon the vehicle being started, several hundred amperes of current flow through and heat the measuring resistor. The temperature rise of the measuring resistor may exceed 100° C. causing considerable thermal expansion of the resistor.
A typical design of a battery current sensing device includes a PCB fixedly connected (i.e., connected via non-flexible leads) to the current sensor. This design is typical as it is economical and can be manufactured easily. However, a problem arises in matching the size of the expanding measuring resistor with the PCB. That is, the non-flexible lead lines do not provide much play to accommodate the size expansion of the measuring resistor relative to the PCB.
As indicated above, a solution to this problem includes making the measuring resistor and the PCB out of materials having similar coefficients of thermal expansion. However, a disadvantage with this solution is that the selection of possible materials to use for both the measuring resistor and the PCB is limited. Further, this solution partially solves the problem as temperature equalization between the measuring resistor and the PCB requires a certain time with the result being that temperature-related mechanical stresses cannot be avoided. Furthermore, the PCB should not assume the temperature of the measuring resistor, which can be quite high, to protect electronic components (e.g., a measuring circuit) on the PCB.